In recent years, a chip antenna for GPS (Global Positioning System) or Bluetooth is built in a small-size portable terminal such as a mobile-phone. A chip antenna of this type is required to be small in size and to facilitate resonance frequency adjustment and impedance matching. This is because the resonance frequency and the input impedance of the chip antenna are affected by the structure of the printed circuit board, various electronic components mounted around the chip antenna, and the housing. Therefore, it is necessary to adjust the resonance frequency and the input impedance for each model.
Particularly, it is very important to facilitate the input impedance adjustment of an antenna for the following reason. When the input impedance does not match a feeder-side impedance, VSWR characteristics of the antenna deteriorate and the antenna cannot exhibit performance inherent in the antenna. To facilitate input impedance matching, Japanese Patent Application Laid-Open No. 11-340726 discloses an antenna device having the following structure. A U-shaped radiation conductor, a ground conductor, and a feeder-to-ground short-circuit conductor are formed on an upper surface of a substrate, a bottom surface thereof, and a side surface thereof, respectively. An inductance value of the feeder-to-ground short-circuit conductor is changed by adjusting a branching point of the feeder-to-ground short-circuit conductor, thereby adjusting an input impedance of the antenna.
Meanwhile, Japanese Patent Application Laid-open No. 2006-340368 discloses an antenna device that can efficiently create an electromagnetic field between an antenna and a ground conductor. This antenna device has a trio-land structure in which a dielectric block is provided in a region surrounded by a ground conductor pattern at three sides, one side of the ground conductor is connected to the opposite side of the ground conductor by the dielectric block, and power is supplied via an input pad provided at the bottom surface of the dielectric block.
However, in the conventional antenna device disclosed in Japanese Patent Application Laid-open No. 11-340726, a ground conductor is formed on the bottom surface of the base, and the antenna device is provided only with the base made of a dielectric material. Therefore, to form a λ/4 radiation conductor required for an antenna operation, there is a need to prepare a sufficiently large base, even with a wavelength shortening effect of the dielectric material being taken into consideration.
The conventional antenna device disclosed in Japanese Patent Application Laid-open No. 2006-340368 has a trio-land structure to efficiently generate an electromagnetic field between a dielectric block and a ground electrode. However, there is a demand for a novel structure to efficiently create an electromagnetic field, other than a trio-land structure. Also, in this antenna device, impedance matching is performed by adjusting the distance between the input pad and the first land pad. As a result, there is a limit to the adjustable impedance range.